i agree with he magnesium thing. a deficiency can definately cause your bowels to tense up. magnesium keeps your muscles well-oiled machines. however, there are several different forms of magnesium out there, and everbody's body preffers a different kind. my body likes magnesium lactate. traci obviously responds to a different kind. you may try a few before you see results.
another consideration could be IBS, irritable bowel syndrome. i don't know all that much about it, but it will cause constipation even when your diet is normal, or you are taking fiber supplements...
.... and that was my two cents.